It is often desirable to separate carbon dioxide components from light hydrocarbons in the processing of gas streams. Certain of these separations are, however, made difficult because of the tendency of mixtures of light hydrocarbons and acid gases to form azeotropes.
One such example can be found in the cryogenic distillative separation of methane from acid gas components described in our copending application, Ser. No. 94,226, filed Nov. 14, 1979. This process is particularly effective for separating methane from high CO.sub.2 -content feed in one distillation column without solids formation. The bottoms product of this process contains carbon dioxide, ethane, and higher hydrocarbons, and it is often desirable to separate the carbon dioxide and ethane components in this bottoms product. Such a separation would produce a useful carbon dioxide product as well as an enriched ethane product which could be used for its heating value or as raw material in many chemical syntheses.
Although highly desirable, the separation of carbon dioxide from ethane by distillation has proven to be a difficult problem in practice. This difficulty is caused by the fact that carbon dioxide and ethane form an azeotrope of approximately two thirds carbon dioxide and one third ethane on a mole basis. For a feed mixture containing ethane and carbon dioxide, this azeotrope tends to form in the upper portion of the column, usually making further separation beyond the azeotrope composition impossible. The common practice of employing two distillation towers operating at different pressures to work around the azeotrope does not help with the carbon dioxide/ethane system because pressure has only minimal effect on the composition of the azeotrope. Because of this, attempts to separate carbon dioxide from ethane by distillation have heretofore resulted in an overhead carbon dioxide stream containing approximately azeotropic amounts of ethane, which are unacceptable in many applicatons.
Ethylene also forms an azeotrope with carbon dioxide. Additionally, it is known that the acid gas hydrogen sulfide forms azeotropes with both ethane and propane. These and other possible azeotropes between acid gases and light hydrocarbons present limitations similar to those described for the carbon dioxide/ethane system when efforts are made to perform distillative separations on such systems.